Any casting which encloses an internal cavity having a transverse dimension greater than the homologous dimension of its opening connecting it with the space outside the casting or in any case in the presence of significant undercuts, which cannot be eliminated prior to the extraction of permanent cores through translations and/or rotations, requires cores or parts thereof that can be destroyed and eliminated after the extraction of the solidified casting from the mold.
For this reason the permanent metallic cores, which generate the cavities for the flow of the coolant of the cylinder blocks of internal combustion engines made by die casting, are extracted through the ceiling of the block and leave large openings which weaken the structure and impose limits on the ignition pressure, thus preventing to increase the torque and efficiency of current engines.
The structures of closed deck cylinder blocks, with particular regard to the cooling cavities of the cylinders, are known and implemented since a long time, as for example described in U.S. Pat. No. 4,686,943. These cylinder blocks are currently obtained from cast iron or aluminum alloy castings, cast by gravity in non-durable or durable molds, with disposable cores generally made of sand.
Numerous methods and materials are known in the art for the construction of these cores, with physical and chemical characteristics very different from each other and different ease of destruction and extraction from the cavity. The process and technology of production of said cores differ greatly from each other and can create significant advantages or disadvantages on the outcome of the casting and on their extraction from it.
The main features of the cores affecting the die casting process are the mechanical characteristics of bending, tensile, compression and erosion resistance; these vary greatly with the manufacturing processes and are often conflicting among them, as for example the mechanical resistance and the ease of destruction and removal.
The process of high pressure die casting (HPDC), currently the most economic for the production of cylinder blocks with open ceiling (so-called “open deck”), uses final pressures on the alloy of many hundreds of bar. In this process, during the filling of the mold, the alloy has a kinetic energy that can generate significant pressures on the walls invested directly by the flow and which can generate very high bending stresses on the cores. Furthermore, significant differences in pressure may occur on the opposite walls of the core immersed in the flow of alloy, with the generation of very high bending stress on the core. Then there are non-negligible local tensile stresses caused by the thermal asymmetry due to the poor thermal conductivity of all non-metallic cores.
The combination of these factors, placing the core under local pressure and bending stresses that are not bearable by the materials that have little resistance to local compression and low tensile strength, easily cause erosion and breakage of the cores and rejection of the castings. This has so far prevented the realization of “closed deck” cylinder blocks with the HPDC process and has prevented the realization of high-efficiency engines.
Cores of ceramic nature, which could withstand the stresses of the process being able to reach higher values of bending strength and erosion resistance, present great difficulties of destruction and extraction from the cavities of the casting, which become greater the higher the mechanical characteristics become, being impossible to make use of easy thermal or dissolution break-up processes. A complex geometric conformation of the cavities, required by structural or functional reasons, could make very difficult and uncertain the complete extraction of the fragments and thus the use of such materials.
The cores obtained from sand and organic or inorganic binders offer good ease of extraction, by mechanical or thermal or combined processes, and have acceptable construction costs and recycling or disposal costs, but have modest mechanical properties which are insufficient to use them with the HPDC process since they offer a strength of a few MPa in bending and a dozen MPa in compression.
Currently under consideration are also die casting processes with cores in mixtures of die cast salts, soluble in water, possibly additioned with inert components, as described in US 2006/01858015, US 2009/0205801, US 2009/0288797, US 2011/0062624 and others. These cores have mechanical properties even far superior to those of the best sand cores, but ecological problems could hinder their application.
In horizontal cold chamber HPDC presses, for functional reasons, the mold must be fed by a casting system, meant as a set of supply ducts of the molten alloy, all outside the envelope of the mold itself, with feeding points of the mold usually far from the area of any disposable cores, as will be explained in the description of the system. This causes a considerable thermal loss of the alloy and implies the need of short filling times of the mold via high speed flows and, consequently, a high injection pressure.
The order of magnitude of the compaction pressure of the casting is several hundreds of bars, reason why every little cavity or crevice of the non-metallic core is filled by the alloy, its penetration being favored by the very low thermal conductivity of the core. This results in a high probability of damage of the cores invested by the flow and, above all, defects in the castings.
However, the considerations above are still the subject of authoritative studies by the major manufacturers of the automotive industry, which has achieved a specific power of light alloy engines no longer improvable with HPDC technology that currently requires all manufacturers to use removable metal cores of sizes equal to those of the cooling cavities, i.e. to adopt cylinder blocks with “open deck” structure with weakened ceiling.
This would be directing the mass production of “closed deck” cylinder blocks towards a process with low-pressure alloy feed to the mold (LPDC=Low Pressure Die Casting), possibly assisted by creating a vacuum in the mold.
This technology, which covers a much smaller fraction than the HPDC process on the total of castings for the automotive sector, is widespread for the production of castings such as medium series of light alloy wheels, as well as for the production of cylinder-heads of the motors that mandatorily require sand cores because of the intricate shapes of the ducts.
In this type of plants the alloy is pushed into the mold of heat resistant steel, placed above the oven, through a large vertical tube having the lower end immersed in the molten alloy, using a slightly compressed gas, usually air, which is then discharged in the atmosphere after the solidification of the casting. In these plants, the overpressure of the feed gas is usually comprised between 0.5 and 1.5 bar for essentially structural reasons, and in any case it could not be of higher orders of magnitude due to the nature and the consumption of the propellant. The process uses therefore pressures of about one order higher than that of the casting by gravity, with times of filling of the impression indicatively of one or two tens of seconds.
Even using the combined technology, with vacuum aspiration and subsequent gas pressure, it would not be easy to achieve the feeding of the solidification shrinkage, especially on parts of the casting far away from the gate. This obliges to have any pouring spout of significant cross-section and of limited length, and, especially, to have casting thicknesses usually higher than necessary. Furthermore the high temperatures of both the molten alloy and the mold involve a low cooling rate, of the order of a few minutes, due also to the considerable molten mass contained in the feed duct.
Additional limitations to the LPDC process result from the strong non-uniformity of the thicknesses that may be required to the casting, since the thin parts distant from the point of supply would cause shrinkage porosity in nearby thicker parts that cool down more slowly, and these may be not fed by the negligible pressure existing at the upper end of the supply duct, due to the viscosity of the alloy which increases while the passages are reduced with the progress of solidification. This often results in unnecessary or detrimental weighting of the castings and, in any case, in low rates of production.